1. Field of the Invention
This invention relates to a control system of a radio telephone apparatus, typically a cordless telephone apparatus, and particularly to transmission control of a paging signal generated from a base unit in response to a call signal sent from a telephone line.
2. Description of the Prior Art
As is well known in the art, a radio telephone apparatus, typically a cordless telephone apparatus is constituted by a radio telephone system having a handset (a handset is also called a radio telephone set) and a base unit connected to wired telephone lines for exchanging speech signals between the wired telephone lines and the radio telephone set through a radio channel. The base unit and the radio telephone set are respectively provided with a pair of corresponding transmitters and receivers. When a call signal arrives at a wired telephone line, a paging signal is transmitted through a predetermined control channel for forming a speech channel from the transmitter of the base unit, thereby to inform the radio telephone set of the fact that it is called from a wired telephone line. Then, the transmitter in the radio telephone set starts operation to send out a paging response signal through the control channel.
The base unit detects the intensity of the received paging response signal sent out from the radio telephone set, i.e., the intensity of the paging response signal at the control channel, and stops the transmission of the paging signal when the detected intensity exceeds a predetermined level, and then switches the radio channel between the base unit and the radio telephone set from the control channel to a speed channel and transmits a bell ringing signal. After transmitting a bell ringing signal, upon receiving a hook off signal from the radio telephone set, the base unit establishes a speech path between the wired telephone line and the radio telephone set.
With the prior art radio telephone apparatus, a speech path is established between the wired telephone line and the radio telephone set according to the procedures described above.
However, as the prior art radio telephone apparatus is constructed such that when the base unit receives at the control channel a carrier wave whose field strength is greater than a predetermined value, it judges that the received carrier wave as a paging response signal so as to stop the transmission of the paging signal. As a consequence, where there are two radio telephone systems (first and second) when call signals simultaneously arrive at the wired telephone line of respective radio telephone systems and, for example, when the first radio telephone set responds to the call signal, the transmission of the paging signal of the base unit associated with the second radio telephone system is also stopped by the paging response signal of the first radio telephone set, whereby the second radio telephone set cannot make any reply. A similar problem also occurs where there is a source of interfacing electric waves in a manufacturing factory, which is similar to the carrier wave transmitted from the radio telephone set, the called radio telephone set cannot respond.